Polarizing element, polarizing element manufacturing method, and optical device

ABSTRACT

Provided is a polarizing element having a wire grid structure, including: a transparent substrate; and grid-shaped convex portions arranged on the transparent substrate at a pitch shorter than a wavelength of light in a use band and extending in a predetermined direction, wherein the grid-shaped convex portion includes a first absorption layer, a first dielectric layer, a reflection layer, a second dielectric layer, and a second absorption layer in order from the transparent substrate side. Further, an optical device including the polarizing plate is provided.

This application is based on and claims the benefit of priority fromJapanese Patent Application No. 2017-185286, filed on 26 Sep. 2017, andU.S. patent application Ser. No. 16/139,973, filed on Sep. 24, 2018,titled “POLARIZING ELEMENT, POLARIZING ELEMENT MANUFACTURING METHOD, ANDOPTICAL DEVICE”. The content of both of these applications isincorporated herein by reference.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a polarizing element, a polarizingelement manufacturing method, and an optical device.

Related Art

A polarizing element is an optical element that absorbs polarized lightin one direction and transmits polarized light in a direction orthogonalto the one direction. In a liquid crystal display device, a polarizingelement is necessary in principle. Particularly, in a liquid crystaldisplay device using a light source with a large light amount such as atransmissive liquid crystal projector, the polarizing element receivesstrong radiation. For this reason, a size of about several centimetersand a high extinction ratio are required in addition to excellent heatresistance. In order to cope with such a demand, a wire grid typeinorganic polarizing element has been proposed.

A wire grid type polarizing element has a structure in which a pluralityof conductor wires (reflection layers) extending in one direction arearranged on a transparent substrate at a pitch (several tens of nm toseveral hundreds of nm) shorter than a wavelength of light in a useband. When light is incident to the polarizing element, polarized light(TE wave (S wave)) parallel to the wire extension direction cannot betransmitted and polarized light (TM wave (P wave)) perpendicular to thewire extension direction is directly transmitted. Since the wire gridtype polarizing element has excellent heat resistance, can manufacture arelatively large element, and has a high extinction ratio, thepolarizing element is suitable for an application such as a liquidcrystal projector.

So far, polarizing elements having various structures have been proposedas the wire grid type polarizing element.

For example, Patent Document 1 discloses a polarizing element includinga base body and grid-shaped convex portions arranged on the base body ata pitch shorter than a wavelength of light in a use band, and thegrid-shaped convex portion includes a wire grid layer, a dielectriclayer, an absorption layer, and a dielectric layer in order from thebase body side. Further, Patent Document 2 discloses a polarizingelement including a transparent substrate and grid-shaped convexportions arranged on the transparent substrate at a pitch shorter than awavelength of light in a use band, and the grid-shaped convex portionincludes a reflection layer, a dielectric layer, and an absorption layerin order from the transparent substrate side.

Patent Document 1: Japanese Patent No. 5184624

Patent Document 2: Japanese Patent No. 5960319

SUMMARY OF THE INVENTION

Incidentally, when the wire grid type polarizing element is used in theliquid crystal projector, the grid surface provided with the grid-shapedconvex portion is normally disposed toward a liquid crystal panel. Thus,in the polarizing element disposed on the light emission side of theliquid crystal panel, light is incident from the grid surface sideprovided with the grid-shaped convex portion. According to theconventional polarizing element disclosed in Patent Documents 1 and 2,it is possible to suppress the absorption axis reflectance to be low forthe light incident from the grid surface side provided with thegrid-shaped convex portion.

However, in the liquid crystal projector, there is a case in which thelight transmitted through the polarizing element on the light emissionside is reflected by the other optical elements or the like and isincident to the polarizing element as returned light. In this case, inthe conventional polarizing element disclosed in Patent Documents 1 and2, the absorption axis reflectance cannot be suppressed to be low andreflection occurs at a high ratio. If such reflection of returned lightis repeated, image quality may be deteriorated due to ghost or the like.

The invention has been made in view of the above-described circumstancesand an object thereof is to provide a polarizing element capable ofsuppressing an absorption axis reflectance to be low for both of lightincident from a grid surface side provided with a grid-shaped convexportion and light incident from a substrate surface side, a polarizingelement manufacturing method, and an optical device including thepolarizing element.

In order to attain the above-described object, the invention provides apolarizing element (for example, a polarizing element 1 to be describedlater) having a wire grid structure, including: a transparent substrate(for example, a transparent substrate 10 to be described later); andgrid-shaped convex portions (for example, grid-shaped convex portions 11to be described later) arranged on the transparent substrate at a pitchshorter than a wavelength of light in a use band and extending in apredetermined direction, in which each grid-shaped convex portionincludes a first absorption layer (for example, a first absorption layer13 to be described later), a first dielectric layer (for example, afirst dielectric layer 14 to be described later), a reflection layer(for example, a reflection layer 15 to be described later), a seconddielectric layer (for example, a second dielectric layer 16 to bedescribed later), and a second absorption layer (for example, a secondabsorption layer 17 to be described later) in order from the transparentsubstrate side.

The grid-shaped convex portion may include a base (for example, a base12 to be described later) between the transparent substrate and thefirst absorption layer and the base may have a trapezoidal shape asviewed from the predetermined direction.

The base may be formed of Si oxide which is transparent to thewavelength of the light in the use band.

The first absorption layer and the second absorption layer may be formedof the same material.

The first dielectric layer and the second dielectric layer may be formedof the same material.

A film thickness of the first absorption layer may be substantially thesame as a film thickness of the second absorption layer and a filmthickness of the first dielectric layer may be substantially the same asa film thickness of the second dielectric layer.

The transparent substrate may be transparent to the wavelength of thelight in the use band and may be formed of glass, quartz, or sapphire.

The reflection layer may be formed of aluminum or an aluminum alloy.

The first dielectric layer and the second dielectric layer may be formedof Si oxide.

The first absorption layer and the second absorption layer may includeFe and Ta and may further include Si.

A surface on the side of the grid-shaped convex portion of thepolarizing element may be covered with a protection film formed of adielectric material.

A surface on the side of the grid-shaped convex portion of thepolarizing element may be covered with an organic water repellent film.

A grid tip (for example, a grid tip 19 to be described later) formed ata tip of the grid-shaped convex portion may have a taper shape of whicha side surface is inclined so that a width is narrowed toward the tip asviewed from the predetermined direction.

The reflection layer may include a metal layer (for example, a metallayer 151 to be described later) and an oxide layer (for example, anoxide layer 152 to be described later) formed of metal oxide forming themetal layer and covering a side surface of the metal layer as viewedfrom the predetermined direction.

A width of the reflection layer may be smaller than those of the firstdielectric layer and the second dielectric layer.

Further, the invention provides a method of manufacturing a polarizingelement having a wire grid structure, including: forming a laminationstructure including a first absorption layer, a first dielectric layer,a reflection layer, a second dielectric layer, and a second absorptionlayer in this order from the transparent substrate side on thetransparent substrate; and forming grid-shaped convex portions arrangedon the transparent substrate at a pitch shorter than a wavelength oflight in a use band by selectively etching the lamination structure.

Further, the invention provides an optical device including thepolarizing element.

According to the invention, it is possible to provide a polarizingelement capable of suppressing an absorption axis reflectance to be lowfor both of light incident from a grid surface side provided with agrid-shaped convex portion and light incident from a substrate surfaceside, a polarizing element manufacturing method, and an optical deviceincluding the polarizing element.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view illustrating an example of anoptical element according to the embodiment.

FIG. 2 is a schematic cross-sectional view illustrating an opticalelement according to Modified Example 1 of the embodiment.

FIG. 3 is a schematic cross-sectional view illustrating an opticalelement according to Modified Example 2 of the embodiment.

FIG. 4 is a schematic cross-sectional view illustrating an opticalelement according to Modified Example 3 of the embodiment.

FIG. 5 is a schematic cross-sectional view illustrating an example of anoptical element without a first absorption layer and a first dielectriclayer.

FIG. 6 is a graph showing a result obtained by verifying an absorptionaxis reflectance of light incident from a grid surface side in apolarizing element having a structure illustrated in FIG. 1 and apolarizing element having a structure illustrated in FIG. 5 bysimulation.

FIG. 7 is a graph showing a result obtained by verifying an absorptionaxis reflectance of light incident from a substrate surface side in thepolarizing element having a structure illustrated in FIG. 1 and thepolarizing element having a structure illustrated in FIG. 5 bysimulation.

FIG. 8 is a graph showing a result obtained by verifying a transmissionaxis transmittance, an absorption axis transmittance, a transmissionaxis reflectance, and an absorption axis reflectance after thepolarizing element having a structure illustrated in FIG. 1 is actuallymanufactured.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, an embodiment of the invention will be described in detailwith reference to the drawings.

[Polarizing Element]

A polarizing element according to the embodiment is a polarizing elementhaving a wire grid structure, including: a transparent substrate andgrid-shaped convex portions arranged on the transparent substrate at apitch (period) shorter than a wavelength of light in a use band andextending in a predetermined direction. Further, each grid-shaped convexportion includes a first absorption layer, a first dielectric layer, areflection layer, a second dielectric layer, and a second absorptionlayer in order from the transparent substrate side.

FIG. 1 is a schematic cross-sectional view illustrating an example of apolarizing element 1 according to the embodiment. As illustrated in FIG.1, the polarizing element 1 includes a transparent substrate 10 which istransparent to the light in the use band and grid-shaped convex portions11 which are arranged on one surface of the transparent substrate 10 ata pitch P shorter than the wavelength of the light in the use band. Eachgrid-shaped convex portion 11 includes a base 12 formed as necessary, afirst absorption layer 13, a first dielectric layer 14, a reflectionlayer 15, a second dielectric layer 16, and a second absorption layer 17in order from the side of the transparent substrate 10. That is, thepolarizing element 1 illustrated in FIG. 1 has a wire grid structure inwhich the grid-shaped convex portions 11 formed by stacking the base 12,the first absorption layer 13, the first dielectric layer 14, thereflection layer 15, the second dielectric layer 16, and the secondabsorption layer 17 in order from the side of the transparent substrate10 are arranged on the transparent substrate 10 in a one-dimensionallattice shape.

In the specification, as illustrated in FIG. 1, the extension direction(a predetermined direction) of the grid-shaped convex portion 11 will bereferred to as the Y-axis direction. Further, a direction which isorthogonal to the Y-axis direction and in which the grid-shaped convexportions 11 are arranged along the main surface of the transparentsubstrate 10 will be referred to as the X-axis direction. In this case,the light incident to the polarizing element 1 is desirably incidentfrom a direction orthogonal to the X-axis direction and the Y-axisdirection at the side (the grid surface side) provided with thegrid-shaped convex portion 11 of the transparent substrate 10.

By using four actions of transmission, reflection, interference, andselective light absorption of polarization waves due to opticalanisotropy, the polarizing element 1 attenuates polarized light havingan electric field component parallel to the Y-axis direction (TE wave (Swave)) and transmits polarized light (TM wave (P wave)) having anelectric field component parallel to the X-axis direction. Thus, theY-axis direction is the direction of the absorption axis of thepolarizing element 1 and the X-axis direction is the direction of thetransmission axis of the polarizing element 1.

The light incident from the side (the grid surface side) provided withthe grid-shaped convex portion 11 of the polarizing element 1 isattenuated while a part of the light is absorbed when the light passesthrough the second absorption layer 17 and the second dielectric layer16. Among the light transmitted through the second absorption layer 17and the second dielectric layer 16, the TM wave (P wave) is transmittedthrough the reflection layer 15, the first dielectric layer 14, and thefirst absorption layer 13 at a high transmittance. Meanwhile, among thelight transmitted through the second absorption layer 17 and the seconddielectric layer 16, the TE wave (S wave) is reflected by the reflectionlayer 15. When the TE wave which is reflected by the reflection layer 15passes through the second dielectric layer 16 and the second absorptionlayer 17, a part of the wave is absorbed and a part of the wave isreflected and returned to the reflection layer 15. Further, the TE wavereflected by the reflection layer 15 is attenuated by interference atthe time of passing through the second dielectric layer 16 and thesecond absorption layer 17.

Meanwhile, the light incident from the side of the transparent substrate10 (the substrate surface side) of the polarizing element 1 isattenuated while a part of the light is absorbed when the light passesthrough the first absorption layer 13 and the first dielectric layer 14.Among the light transmitted through the first absorption layer 13 andthe first dielectric layer 14, TM wave (P wave) is transmitted throughthe reflection layer 15, the second dielectric layer 16, and the secondabsorption layer 17 at a high transmittance. Meanwhile, among the lighttransmitted through the first absorption layer 13 and the firstdielectric layer 14, TE wave (S wave) is reflected by the reflectionlayer 15. When the TE wave reflected by the reflection layer 15 passesthrough the first dielectric layer 14 and the first absorption layer 13,a part of the wave is absorbed and a part of the wave is reflected andreturned to the reflection layer 15. Further, the TE wave reflected bythe reflection layer 15 is attenuated by interference at the time ofpassing through the first dielectric layer 14 and the first absorptionlayer 13.

As described above, in the polarizing element 1 according to theembodiment, it is possible to suppress the absorption axis reflectanceto be low for both of the light incident from the grid surface sideprovided with the grid-shaped convex portion and the light incident fromthe substrate surface side.

The transparent substrate 10 is not particularly limited as long as thetransparent substrate 10 is a substrate exhibiting translucency to thelight in the use band and can be appropriately selected in accordancewith the purpose. The phrase “exhibiting translucency to the light inthe use band” does not denote that the transmittance of the light in theuse band is 100%, and the phrase may denote that the transparentsubstrate may exhibit translucency capable of retaining a function as apolarizing element. As the light in the use band, for example, visiblelight having a wavelength of about 380 nm to 810 nm can be exemplified.

The shape of the main surface of the transparent substrate 10 is notparticularly limited and a shape (for example, a rectangular shape)according to a purpose is appropriately selected. The average thicknessof the transparent substrate 10 is desirably, for example, 0.3 mm to 1mm.

As the material constituting the transparent substrate 10, a materialhaving a refractive index of 1.1 to 2.2 is desirable and glass, quartz,sapphire, and the like can be exemplified. From the viewpoint of costand light transmittance, it is desirable to use glass, particularly,quartz glass (having a refractive index of 1.46) or soda lime glass(having a refractive index of 1.51). The composition of the element ofthe glass material is not particularly limited and, for example, aninexpensive glass material such as silicate glass widely distributed asan optical glass can be used.

Further, from the viewpoint of thermal conductivity, it is desirable touse quartz or sapphire having high thermal conductivity. Accordingly,since high light fastness with respect to strong light is obtained, itis desirably used as a polarizing element for an optical engine of aprojector with a large heat generation amount.

Additionally, when a transparent substrate formed of optically activecrystals such as quartz is used, it is desirable to arrange thegrid-shaped convex portion 11 in a direction parallel or perpendicularto the optical axis of the crystal. Accordingly, excellent opticalcharacteristics can be obtained. Here, the optical axis indicates adirection axis in which a difference in refractive index between thelight O (normal light) traveling in that direction and the light E(ideal light) becomes minimal.

The grid-shaped convex portions 11 are arranged on the transparentsubstrate at a pitch P shorter than the wavelength of the light in theuse band. There is no particular limitation as long as the pitch P ofthe grid-shaped convex portion 11 is shorter than the wavelength of thelight in the use band. From the viewpoint of ease of production andstability, the pitch P of the grid-shaped convex portions 11 isdesirably, for example, 100 nm to 200 nm. The pitch P of the grid-shapedconvex portion 11 can be measured by the observation using a scanningelectron microscope or a transmission electron microscope. For example,the pitches of four arbitrary positions are measured by using thescanning electron microscope or the transmission electron microscope andan arithmetic average value thereof can be set to the pitch of thegrid-shaped convex portions 11. Hereinafter, this measurement methodwill be referred to as an electron microscopy method.

The width W of the grid-shaped convex portion 11 is not particularlylimited, but is desirably smaller than the width of the concave portionbetween the grid-shaped convex portions 11. Specifically, the width W ofthe grid-shaped convex portion 11 is desirably, for example, 35 nm to 45nm. The width W of the grid-shaped convex portion 11 can be measured bythe above-described electron microscopy method at the center position ofthe height of the grid-shaped convex portion 11.

The base 12 has a trapezoidal shape as viewed from the extensiondirection of the grid-shaped convex portion 11 (a predetermineddirection), that is, as viewed from a cross-section orthogonal to thepredetermined direction as illustrated in FIG. 1. More specifically, thebase 12 has an isosceles trapezoidal shape in which the side surface isinclined so that the width decreases from the side of the transparentsubstrate 10 toward the side of the first absorption layer 13 as viewedfrom a predetermined direction.

The film thickness of the base 12 is not particularly limited and isdesirably, for example, 10 nm to 100 nm. Additionally, the filmthickness of the base 12 can be measured by, for example, theabove-described electron microscopy method.

The base 12 has a configuration in which the dielectric film extendingin a band shape in the Y-axis direction corresponding to the absorptionaxis is arranged on the transparent substrate 10. As the materialforming the base 12, a material which is transparent to the light in theuse band and has a refractive index smaller than that of the transparentsubstrate 10 is desirable and particularly, Si oxide such as SiO₂ isdesirable.

The base 12 can be formed by gradually changing a balance betweenisotropic etching and anisotropic etching by dry etching with respect tothe underlying layer 18 made of the above-described dielectric andformed on the transparent substrate 10. In this case, as illustrated inFIG. 1, the base 12 is disposed on the underlying layer 18 formed on thetransparent substrate 10. Since the base 12 is formed in a trapezoidalshape, it is considered that the same effect as that of the moth eyestructure of which the refractive index gently changes can be obtained,the reflection of light can be prevented, and the high transmittancecharacteristics can be obtained.

Here, as described above, in the embodiment, the base 12 is not anessential configuration and the polarizing element 1 may not include thebase 12. In this case, the grid-shaped convex portions 11 can bedirectly arranged on the underlying layer 18.

The first absorption layer 13 is formed on the base 12 and is arrangedto extend in a band shape in the Y-axis direction corresponding to theabsorption axis. As the material forming the first absorption layer 13,one or more materials (a metal material, a semiconductor material, andthe like) having a light absorbing function in which the extinctionconstant of the optical constant is not zero can be exemplified, and thematerial is appropriately selected depending on the wavelength range oflight to be applied. As the metal material, an element such as Ta, Al,Ag, Cu, Au, Mo, Cr, Ti, W, Ni, Fe, and Sn or an alloy including at leastone element thereof can be exemplified. Further, as the semiconductormaterial, Si, Ge, Te, ZnO, silicide materials (β-FeSi₂, MgSi₂, NiSi₂,BaSi₂, CrSi₂, CoSi₂, TaSi, and the like) can be exemplified. When thesematerials are used, the polarizing element 1 can have a high extinctionratio with respect to the visible light range to be applied. Amongthese, the first absorption layer 13 desirably includes Fe or Ta andfurther includes Si.

When the semiconductor material is used as the first absorption layer13, since the band gap energy of the semiconductor is involved with theabsorption function, it is necessary that the band gap energy is equalto or smaller than the use band. For example, in the case of applicationto the visible light, it is necessary to use a material havingabsorption at a wavelength of 400 nm or more, that is, a material havinga band gap of 3.1 eV or less.

The film thickness of the first absorption layer 13 is not particularlylimited and is desirably, for example, 10 nm to 100 nm. The filmthickness of the first absorption layer 13 can be measured by, forexample, the above-described electron microscopy method. Additionally,the first absorption layer 13 can be formed as a high-density film byvapor deposition, sputtering, or the like. Further, the first absorptionlayer 13 may include two or more layers having different formingmaterials.

The first dielectric layer 14 is formed on the first absorption layer 13and is formed by arranging a dielectric film extending in a band shapein the Y-axis direction corresponding to the absorption axis. The firstdielectric layer 14 is formed to have a film thickness which allows thetransmission in the first absorption layer 13 with respect to thepolarized light incident from the substrate surface side and reflectedby the first absorption layer 13 and shifts the phase of the polarizedlight reflected by the reflection layer 15 by a half wavelength.Specifically, the film thickness of the first dielectric layer 14 isappropriately set in the range of 1 nm to 500 nm capable of improving aninterference effect by adjusting the phase of the polarized light. Thefilm thickness of the first dielectric layer 14 can be measured by, forexample, the above-described electron microscopy method.

As the material forming the first dielectric layer 14, a generalmaterial such as Si oxide such as SiO₂, metal oxides such as Al₂O₃,beryllium oxide, and bismuth oxide, MgF₂, cryolite, germanium, titaniumdioxide, silicon, magnesium fluoride, boron nitride, boron oxide,tantalum oxide, carbon, or a combination of these can be exemplified.Among these, the first dielectric layer 14 is desirably formed of Sioxide.

The refractive index of the first dielectric layer 14 is desirablylarger than 1.0 and equal to or smaller than 2.5. Since the opticalcharacteristics of the reflection layer 15 are also influenced by theperipheral refractive index, it is possible to control thecharacteristics of the polarizing element 1 by selecting the material ofthe first dielectric layer 14. Further, since the film thickness and therefractive index of the first dielectric layer 14 are appropriatelyadjusted, when the TE wave incident from the substrate surface side andreflected by the reflection layer 15 is transmitted through the firstabsorption layer 13, a part of the wave can be reflected and returned tothe reflection layer 15 and the light passing through the firstabsorption layer 13 can be attenuated by interference. In this way,since the TE wave is selectively attenuated among the light incidentfrom the substrate surface side, it is possible to obtain desiredpolarization characteristics.

The reflection layer 15 is formed on the first dielectric layer 14 andis formed by arranging a metal film extending in a band shape in theY-axis direction corresponding to the absorption axis. The reflectionlayer 15 has a function as a wire grid type polarizer and is used toattenuate the polarized wave having an electric field component in adirection parallel to the longitudinal direction of the reflection layer15 (TE wave (S wave)) and transmit the polarized wave having an electricfield component in a direction orthogonal to the longitudinal directionof the reflection layer 15 (TM wave (P wave)).

The material forming the reflection layer 15 is not particularly limitedas long as the material is reflective to the light in the use band and,for example, an element such as Al, Ag, Cu, Mo, Cr, Ti, Ni, W, Fe, Si,Ge, and Te or an alloy including one or more elements thereof can beexemplified. Among these, the reflection layer 15 is desirably formed ofaluminum or an aluminum alloy. Additionally, in addition to these metalmaterials, the reflection layer 15 may be formed by, for example, aninorganic film or a resin film formed with high surface reflectance dueto coloring or the like other than metal.

The film thickness of the reflection layer 15 is not particularlylimited and is desirably, for example, 100 nm to 300 nm. Additionally,the film thickness of the reflection layer 15 can be measured by, forexample, the above-described electron microscopy method.

The second dielectric layer 16 is formed on the reflection layer 15 andis formed by arranging the dielectric film extending in a band shape inthe Y-axis direction corresponding to the absorption axis. The seconddielectric layer 16 is formed to have a film thickness which allows thetransmission in the second absorption layer 17 with respect to thepolarized light incident from the grid surface side and reflected by thesecond absorption layer 17 and shifts the phase of the polarized lightreflected by the reflection layer 15 by a half wavelength. Specifically,the film thickness of the second dielectric layer 16 is appropriatelyset in the range of 1 nm to 500 nm capable of improving an interferenceeffect by adjusting the phase of the polarized light. The film thicknessof the second dielectric layer 16 can be measured by, for example, theabove-described electron microscopy method.

As the material forming the second dielectric layer 16, the samematerial as that of the first dielectric layer 14 can be exemplified.Particularly, the second dielectric layer 16 is desirably formed of thesame material as that of the first dielectric layer 14. When the firstdielectric layer 14 and the second dielectric layer 16 are formed of thesame material, an etching condition and the like at the time ofmanufacturing can be the same and thus the manufacturing can befacilitated. Further, the performance of the first dielectric layer 14and the performance of the second dielectric layer 16 can be matched.

The refractive index of the second dielectric layer 16 is desirablylarger than 1.0 and is equal to or smaller than 2.5. Since the opticalcharacteristics of the reflection layer 15 are also influenced by theperipheral refractive index, the characteristics of the polarizingelement 1 can be controlled by selecting the material of the seconddielectric layer 16. Further, since the film thickness and therefractive index of the second dielectric layer 16 are appropriatelyadjusted, when the TE wave incident from the grid surface side andreflected by the reflection layer 15 is transmitted through the secondabsorption layer 17, a part of the wave can be reflected and returned tothe reflection layer 15 and the light passing through the secondabsorption layer 17 can be attenuated by interference. In this way,since the TE wave is selectively attenuated among the light incidentfrom the grid surface side, it is possible to obtain desiredpolarization characteristics.

The second absorption layer 17 is formed on the second dielectric layer16 and is arranged to extend in a band shape in the Y-axis directioncorresponding to the absorption axis. As the material forming the secondabsorption layer 17, the same material as that of the first absorptionlayer 13 can be exemplified. Particularly, the second absorption layer17 is desirably formed of the same material as that of the firstabsorption layer 13. When the first absorption layer 13 and the secondabsorption layer 17 are formed of the same material, an etchingcondition and the like at the time of manufacturing can be the same andthus the manufacturing can be facilitated. Further, the performance ofthe first absorption layer 13 and the performance of the secondabsorption layer 17 can be matched.

The film thickness of the second absorption layer 17 is not particularlylimited and is desirably, for example, 10 nm to 100 nm. The filmthickness of the second absorption layer 17 can be measured by, forexample, the above-described electron microscopy method. Additionally,the second absorption layer 17 can be formed as a high-density film byvapor deposition, sputtering, or the like. Further, the secondabsorption layer 17 may include two or more layers having differentforming materials.

Additionally, in the polarizing element 1 according to the embodiment,it is desirable that the film thickness of the first absorption layer 13be substantially the same as the film thickness of the second absorptionlayer 17 and the film thickness of the first dielectric layer 14 besubstantially the same as the film thickness of the second dielectriclayer 16. Specifically, when the film thickness of the first absorptionlayer 13 is indicated by t₁ (nm), the film thickness of the secondabsorption layer 17 is desirably 0.90 t₁ (nm) to 1.10 t₁ (nm) and moredesirably 0.95 t₁ (nm) to 1.05 t₁ (nm). Further, when the film thicknessof the first dielectric layer 14 is indicated by t₂ (nm), the filmthickness of the second dielectric layer 16 is desirably 0.90 t₂ (nm) to1.10 t₂ (nm) and more desirably 0.95 t₂ (nm) to 1.05 t₂ (nm). When thefilm thickness is set to be the same in this way, it is possible tomatch the wavelength at the minimum point of the absorption axisreflectance for the light incident from the grid surface side and thelight incident from the substrate surface side.

The polarizing element 1 according to the embodiment with theabove-described configuration may have a diffusion barrier layer at aposition between the first absorption layer 13 and the first dielectriclayer 14 and at a position between the second dielectric layer 16 andthe second absorption layer 17. That is, in this case, the grid-shapedconvex portion 11 includes the base 12, the first absorption layer 13,the diffusion barrier layer, the first dielectric layer 14, thereflection layer 15, the second dielectric layer 16, the diffusionbarrier layer, and the second absorption layer 17 in order from the sideof the transparent substrate 10. When the polarizing element 1 has adiffusion barrier layer, the diffusion of light in the first absorptionlayer 13 and the second absorption layer 17 can be prevented. Thediffusion barrier layer is formed as a metal film of Ta, W, Nb, Ti, orthe like.

Further, the polarizing element 1 according to the embodiment may have aconfiguration in which a surface on the side of a grid surface iscovered with a protection film formed of a dielectric material in arange that does not give an influence on a change in the opticalcharacteristics. The protection film is formed as a dielectric film andcan be formed by using a CVD method (chemical vapor deposition method),an ALD method (atomic layer deposition method), or the like for thesurface on the side of the grid surface. Accordingly, it is possible tosuppress an oxidation reaction more than necessary for the metal film.

Further, the polarizing element 1 according to the embodiment may have aconfiguration in which the surface on the side of the grid surface iscovered with an organic water repellent film. The organic waterrepellent film is formed of a fluorine-based silane compound such asperfluorodecyltriethoxysilane (FDTS) and can be formed by using theabove-described CVD method, ALD method, or the like. Accordingly,reliability such as moisture resistance of the polarizing element 1 canbe improved.

[Polarizing Element Manufacturing Method]

The polarizing element manufacturing method according to the embodimentis a method of manufacturing a polarizing element having a wire gridstructure, including: forming a lamination structure including a firstabsorption layer, a first dielectric layer, a reflection layer, a seconddielectric layer, and a second absorption layer in this order from thetransparent substrate side on the transparent substrate; and forminggrid-shaped convex portions arranged on the transparent substrate at apitch shorter than a wavelength of light in a use band by selectivelyetching the lamination structure.

Hereinafter, as an example, a method of manufacturing the polarizingelement 1 illustrated in FIG. 1 will be described.

First, a lamination structure including an underlying layer, a firstabsorption layer, a first dielectric layer, a reflection layer, a seconddielectric layer, and a second absorption layer formed in this orderfrom the side of the transparent substrate 10 is formed on thetransparent substrate 10. As a method of forming the layers, asputtering method, a vapor deposition method, and the like can beexemplified.

Next, one dimensional lattice mask pattern is formed on the secondabsorption layer by a photolithography method, a nanoimprinting method,and the like. Then, the lamination structure is selectively etched toform the grid-shaped convex portions 11 arranged on the transparentsubstrate 10 at a pitch shorter than the wavelength of the light in theuse band. As the etching method, for example, a dry etching method usingan etching gas corresponding to an etching target can be exemplified.

Particularly, when manufacturing the polarizing element 1 illustrated inFIG. 1, the base 12 having a trapezoidal shape as viewed from theextension direction of the grid-shaped convex portion 11 is formed byoptimizing an etching condition of the underlying layer. With theabove-described configuration, the polarizing element 1 illustrated inFIG. 1 is manufactured.

Additionally, the polarizing element manufacturing method according tothe embodiment may further include coating the surface on the side ofthe grid surface with a protection film. Further, the polarizing elementmanufacturing method according to the embodiment may further includecoating the surface on the side of the grid surface with an organicwater repellent film.

[Optical Device]

An optical device according to the embodiment includes theabove-described polarizing element according to the embodiment. As theoptical device, a liquid crystal projector, a head-up display, a digitalcamera, and the like can be exemplified. Since the polarizing elementaccording to the embodiment is an inorganic polarizing element havingexcellent heat resistance compared to an organic polarizing element, theapplication to the liquid crystal projector, the head-up display, andthe like requiring heat resistance is desirable.

When the optical device according to the embodiment includes a pluralityof polarizing elements, at least one of the plurality of polarizingelements may be the polarizing element according to the embodiment. Forexample, when the optical device according to the embodiment is a liquidcrystal projector, at least one of the polarizing elements disposed onthe light incident side and the light emission side of the liquidcrystal panel may be the polarizing element according to the embodiment.From the viewpoint of further reducing deterioration of image qualitydue to ghost or the like, at least the polarizing element on the lightemission side is desirably the polarizing element according to theembodiment and the polarizing elements on both of the light incidentside and the light emission side are more desirably the polarizingelement according to the embodiment.

According to the polarizing element 1, the polarizing elementmanufacturing method, and the optical device described above, thefollowing effects are obtained.

Since the polarizing element 1 according to the embodiment includes thegrid-shaped convex portion 11 formed by laminating the first absorptionlayer 13, the first dielectric layer 14, the reflection layer 15, thesecond dielectric layer 16, and the second absorption layer 17 in thisorder from the side of the transparent substrate 10, it is possible tosuppress the absorption axis reflectance to be low for both of the lightincident from the grid surface side and the light incident from thesubstrate surface side. Thus, even when the polarizing element 1according to the embodiment is used as the polarizing element on thelight emission side of the liquid crystal panel in the liquid crystalprojector, it is possible to reduce deterioration of image quality dueto ghost or the like.

Additionally, the invention is not limited to the above-describedembodiment and modification and improvement within the scope ofachieving the object of the invention are included in the invention.

FIG. 2 is a schematic cross-sectional view illustrating a polarizingelement 1A according to Modified Example 1 of the embodiment. In thepolarizing element 1A, a grid tip 19 formed at a tip of the grid-shapedconvex portion 11A has a taper shape of which a side surface is inclinedas a width is narrowed toward the tip when viewed from the extensiondirection of the grid-shaped convex portion 11A (the predetermineddirection). More specifically, the grid tip 19 of the polarizing element1A according to Modified Example 1 has an isosceles trapezoidal shape.The grid tip 19 is formed by a part of a reflection layer 15A, a seconddielectric layer 16A, and a second absorption layer 17A.

When the grid tip 19 is formed in a taper shape as illustrated in FIG.2, it is possible to improve the transmittance of the polarized light(TM wave) in the transmission axis direction (the X-axis direction). Thereason why the transmittance of the TM wave increases in this way is asbelow. When the grid tip 19 is formed in a taper shape, there is aneffect of suppressing the scattering of light incident with a variationin angle.

Additionally, in FIG. 2, a part of the reflection layer 15A is includedin the grid tip 19, but the invention is not limited to this structure.For example, the grid tip 19 may be formed only by the second dielectriclayer 16A and the second absorption layer 17A.

FIG. 3 is a schematic cross-sectional view illustrating a polarizingelement 1B according to Modified Example 2 of the embodiment. In thepolarizing element 1B, the reflection layer 15B includes a metal layer151 and an oxide layer 152 formed of metal oxide forming the metal layer151 and covering a side surface of the metal layer 151 as viewed fromthe extension direction of the grid-shaped convex portion 11B (thepredetermined direction).

The material forming the metal layer 151 is not particularly limited aslong as the material is reflective to the light in the use band and, forexample, an element such as Al, Ag, Cu, Mo, Cr, Ti, Ni, W, Fe, Si, Ge,and Te or an alloy including one or more elements thereof can beexemplified. Among these, the metal layer 151 is desirably formed ofaluminum or an aluminum alloy.

The oxide layer 152 is formed of metal oxide forming the metal layer151. For example, when the metal layer 151 is formed of Al, the oxidelayer 152 is formed of Al₂O₃. The oxide layer 152 is formed by anoxidization reaction or the like according to the heat treatment of themetal layer.

When the reflection layer 15B is formed by the metal layer 151 and theoxide layer 152 as illustrated in FIG. 3, the area of the reflectionlayer 15B as viewed from the light incident direction is changed and theamount of light reflected by the reflection layer 15B is changed. Thus,it is possible to obtain the same light transmission characteristics asthose of a case in which the grid width is narrowed without narrowingthe grid width.

Additionally, also in the polarizing element 1B according to ModifiedExample 2, a grid tip may be formed at a tip of the grid-shaped convexportion 11B similarly to the polarizing element 1A according to ModifiedExample 1.

FIG. 4 is a schematic cross-sectional view illustrating a polarizingelement 1C according to Modified Example 3 of the embodiment. In agrid-shaped convex portion 11C of the polarizing element 1C, the widthof the reflection layer 15C is smaller than those of the firstdielectric layer 14 and the second dielectric layer 16.

When the width of the reflection layer 15C is set to be smaller thanthose of the first dielectric layer 14 and the second dielectric layer16 as illustrated in FIG. 4, the area of the reflection layer 15C asviewed from the light incident direction is changed and the amount oflight reflected by the reflection layer 15C is changed. Thus, it ispossible to control the light transmission characteristics of thepolarizing element 1C by controlling the width of the reflection layer15C.

Additionally, also in the polarizing element 1C according to ModifiedExample 3, a grid tip may be formed at a tip of the grid-shaped convexportion 11C similarly to the polarizing element 1A according to ModifiedExample 1.

EXAMPLES

Next, examples of the invention will be described, but the invention isnot limited to these examples.

Example 1 and Comparative Example 1

In Example 1, the polarizing element 1 having a structure illustrated inFIG. 1 was subjected to a simulation. Further, in Comparative Example 1,a polarizing element 100 having a structure illustrated in FIG. 5 wassubjected to a simulation. More specifically, the opticalcharacteristics of the polarizing elements were verified byelectromagnetic field simulation according to a rigorous coupled waveanalysis (RCWA) method. In the simulation, Grating simulator Gsolvermanufactured from Grating Solver Development was used. Additionally, thepolarizing element 1 of Example 1 and the polarizing element 100 ofComparative Example 1 are designed to be optimal to the light of thecolor band (wavelength λ=520 nm to 590 nm (predetermined wavelength)).

FIG. 5 is a schematic cross-sectional view illustrating a structure ofthe polarizing element 100 of Comparative Example 1. In FIG. 5, the samereference numerals are given to the components which are common to thepolarizing element 1 illustrated in FIG. 1. A grid-shaped convex portion101 of the polarizing element 100 has the same configuration as that ofthe grid-shaped convex portion 11 of the polarizing element 1illustrated in FIG. 1 except that the first absorption layer 13 and thefirst dielectric layer 14 are not provided.

FIG. 6 is a graph showing a result obtained by verifying the absorptionaxis reflectance of the light incident from the grid surface side in thepolarizing element 1 having a structure illustrated in FIG. 1 and thepolarizing element 100 having a structure illustrated in FIG. 5 bysimulation. In FIG. 6, a horizontal axis indicates the wavelength A (nm)and a vertical axis indicates the absorption axis reflectance (%). Here,the absorption axis reflectance means the reflectance of the polarizedlight (TE wave) incident to the polarizing element in the absorptionaxis direction (the Y-axis direction).

When light was incident from the grid surface side as shown in FIG. 6,the absorption axis reflectance was suppressed to be low by thefunctions of the second dielectric layer 16 and the second absorptionlayer 17 in any one of the structures of FIG. 1 and FIG. 5.

FIG. 7 is a graph showing a result obtained by verifying the absorptionaxis reflectance of the light incident from the substrate surface sidein the polarizing element 1 having a structure illustrated in FIG. 1 anda polarizing element 100 having a structure illustrated in FIG. 5 bysimulation. In FIG. 7, a horizontal axis indicates the wavelength A (nm)and a vertical axis indicates the absorption axis reflectance (%).

When light was incident from the substrate surface side as shown in FIG.7, the absorption axis reflectance was suppressed to be low by thefunctions of the first absorption layer 13 and the first dielectriclayer 14 in the polarizing element 1 having a structure illustrated inFIG. 1. Meanwhile, since the polarizing element 100 having a structureillustrated in FIG. 5 did not include the first absorption layer 13 andthe first dielectric layer 14, the absorption axis reflectance wasremarkably increased.

Example 2

In Example 2, the polarizing element 1 having a structure illustrated inFIG. 1 was actually manufactured and the optical characteristics wereverified. FIG. 8 is a graph showing a result obtained by verifying thetransmission axis transmittance, the absorption axis transmittance, thetransmission axis reflectance, and the absorption axis reflectance forthe polarizing element 1 having a structure illustrated in FIG. 1. InFIG. 8, a horizontal axis indicates the wavelength A (nm) and a verticalaxis indicates the transmittance or reflectance (%). Here, thetransmission axis transmittance means the transmittance of the polarizedlight (TM wave) incident to the polarizing element in the transmissionaxis direction (the X-axis direction) and the transmission axisreflectance means the reflectance of the polarized light (TM wave)incident to the polarizing element in the transmission axis direction(the X-axis direction). Further, the absorption axis transmittance meansthe transmittance of the polarized light (TE wave) incident to thepolarizing element in the absorption axis direction (the Y-axisdirection). Additionally, the absorption axis reflectance was verifiedfor both of the light incident from the grid surface side and the lightincident from the substrate surface side and the other opticalcharacteristics were verified for the light incident from the gridsurface side.

As shown in FIG. 8, in the polarizing element 1 having a structureillustrated in FIG. 1, it was possible to suppress the absorption axisreflectance to be low for both of the light incident from the gridsurface side and the light incident from the substrate surface side andthere was no bad influence on the other optical characteristics.

EXPLANATION OF REFERENCE NUMERALS

-   1, 1A, 1B, 1C: POLARIZING ELEMENT-   10: TRANSPARENT SUBSTRATE-   11, 11A, 11B, 11C: GRID-SHAPED CONVEX PORTION-   12: BASE-   13: FIRST ABSORPTION LAYER-   14: FIRST DIELECTRIC LAYER-   15, 15A, 15B, 15C: REFLECTION LAYER-   16, 16A: SECOND DIELECTRIC LAYER-   17, 17A: SECOND ABSORPTION LAYER-   18: UNDERLYING LAYER-   19: GRID TIP-   100: POLARIZING ELEMENT-   101: GRID-SHAPED CONVEX PORTION-   151: METAL LAYER-   152: OXIDE LAYER

What is claimed is:
 1. A polarizing element having a wire gridstructure, comprising: a transparent substrate; and grid-shaped convexportions arranged on the transparent substrate at a pitch shorter than awavelength of light in a use band and extending in a predetermineddirection, wherein the grid-shaped convex portion includes a base, afirst absorption layer, a first dielectric layer, a reflection layer, asecond dielectric layer, and a second absorption layer in order from thetransparent substrate, and wherein the base has a trapezoidal shape asviewed in the predetermined direction.
 2. The polarizing elementaccording to claim 1, wherein the base is formed of Si oxide which istransparent to the wavelength of the light in the use band.
 3. Thepolarizing element according to claim 1, wherein the first absorptionlayer and the second absorption layer are formed of the same material.4. The polarizing element according to claim 1, wherein the firstdielectric layer and the second dielectric layer are formed of the samematerial.
 5. The polarizing element according to claim 1, wherein a filmthickness of the first absorption layer is substantially the same as afilm thickness of the second absorption layer and a film thickness ofthe first dielectric layer is substantially the same as a film thicknessof the second dielectric layer.
 6. The polarizing element according toclaim 1, wherein the transparent substrate is transparent to thewavelength of the light in the use band and is formed of glass, quartz,or sapphire.
 7. The polarizing element according to claim 1, wherein thereflection layer is formed of aluminum or an aluminum alloy.
 8. Thepolarizing element according to claim 1, wherein the first dielectriclayer and the second dielectric layer are formed of Si oxide.
 9. Thepolarizing element according to claim 1, wherein the first absorptionlayer and the second absorption layer include Fe or Ta and furtherinclude Si.
 10. The polarizing element according to claim 1, wherein asurface on the side of the grid-shaped convex portion of the polarizingelement is covered with a protection film formed of a dielectricmaterial.
 11. The polarizing element according to claim 1, wherein asurface on the side of the grid-shaped convex portion of the polarizingelement is covered with an organic water repellent film.
 12. Thepolarizing element according to claim 1, wherein a grid tip formed at atip of the grid-shaped convex portion has a taper shape of which a sidesurface is inclined so that a width is narrowed toward the tip as viewedfrom the predetermined direction.
 13. The polarizing element accordingto claim 1, wherein the reflection layer includes a metal layer and anoxide layer formed of metal oxide forming the metal layer and covering aside surface of the metal layer as viewed from the predetermineddirection.
 14. The polarizing element according to claim 1, wherein awidth of the reflection layer is smaller than those of the firstdielectric layer and the second dielectric layer.
 15. A method ofmanufacturing a polarizing element having a wire grid structure,comprising: forming a lamination structure on a transparent substrate,the lamination structure including an underlying layer, a firstabsorption layer, a first dielectric layer, a reflection layer, a seconddielectric layer, and a second absorption layer in this order from thetransparent substrate; and forming grid-shaped convex portions arrangedon the transparent substrate at a pitch shorter than a wavelength oflight in a use band and extending in a predetermined direction, byselectively etching the lamination structure, wherein the grid-shapedconvex portion includes a base between the transparent substrate and thefirst absorption layer, and wherein the base has a trapezoidal shape asviewed in the predetermined direction.
 16. An optical device comprisingthe polarizing element according to claim 1.